13.4 Medical Parasitology: Protozoan and Helminthic Pathogens

Overview of Medical Parasitology

Medical parasitology encompasses single-celled protozoa and multicellular helminths (worms). Parasitic diseases account for significant global morbidity and mortality, characterized by complex lifecycles involving definitive hosts (where sexual reproduction occurs), intermediate hosts (where asexual development occurs), and vectors.

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Protozoan Pathogens

Protozoa are microscopic single-celled eukaryotes classified by anatomical site of infection and locomotion.

Intestinal Protozoa

• Entamoeba histolytica: Acquired by ingesting cysts from fecally contaminated food or water. Trophozoites invade the colonic mucosa, causing flask-shaped intestinal ulcers and bloody diarrhea (amoebic dysentery). Diagnostic hallmark: trophozoites containing ingested red blood cells (erythrophagocytosis). Extraintestinal spread via the portal circulation produces a solitary liver abscess containing "anchovy paste" brown exudate.
• Giardia lamblia: Acquired by ingesting cysts in unfiltered wilderness stream water. Trophozoites possess a characteristic pear shape, two nuclei, flagella, and a ventral sucking disc that attaches to the duodenal mucosa without tissue invasion. Causes malabsorption, abdominal bloating, and foul-smelling, fatty diarrhea (steatorrhea).
• Cryptosporidium parvum: Transmitted via oocysts in water supplies. Oocysts are acid-fast positive. Causes mild self-limited watery diarrhea in immunocompetent hosts, but severe, unremitting, life-threatening watery diarrhea in AIDS patients.

Urogenital Protozoa

• Trichomonas vaginalis: Sexually transmitted flagellated protozoan. Exists only as a trophozoite (no cyst stage). Wet mount microscopy reveals highly motile trophozoites with flagella. Clinical findings in females include severe vaginitis with foul-smelling, frothy green vaginal discharge, intense itching, and a "strawberry cervix" (punctate cervical hemorrhages).

Blood & Tissue Protozoa

• Plasmodium Species (Malaria): Transmitted by the female Anopheles mosquito vector. Microscopic diagnosis relies on thick and thin blood smears.
  • P. falciparum: Most severe form. Shows intracellular ring forms and characteristic banana-shaped/crescentic gametocytes. Causes irregular severe fever spikes, microvascular obstruction, and cerebral malaria.
  • P. vivax & P. ovale: Produce 48-hour tertian fever cycles. Form dormant hypnozoites in liver hepatocytes, requiring treatment with primaquine to prevent relapse.
  • P. malariae: Produces 72-hour quartan fever cycles.
• Toxoplasma gondii: Transmitted by ingesting oocysts from cat feces or tissue cysts in undercooked meat. In immunocompromised (AIDS) patients, reactivation causes encephalitis with multiple ring-enhancing lesions on brain MRI. Congenital toxoplasmosis features the classic triad: chorioretinitis, hydrocephalus, and intracranial calcifications.
• Leishmania donovani: Transmitted by the female sandfly vector. Amastigotes multiply inside host macrophages. Causes visceral leishmaniasis (kala-azar), characterized by massive hepatosplenomegaly, pancytopenia, fever, and hypergammaglobulinemia.
• Trypanosoma cruzi: Transmitted by the Reduviid ("kissing") bug (feces deposited near bite site). Causes Chagas disease. Acute phase shows periorbital swelling (Roma\u00f1a sign). Chronic phase destroys autonomic myenteric plexus, leading to dilated cardiomyopathy, megacolon, and megaesophagus.
• Trypanosoma brucei: Transmitted by the Tsetse fly. Causes African sleeping sickness (somnolence, coma). Demonstrates continuous antigenic variation of its Variant Surface Glycoprotein (VSG) coat, resulting in recurrent waves of parasitemia and fever.

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Helminthic Pathogens: Nematodes (Roundworms)

Nematodes are non-segmented roundworms with complete digestive tracts.

• Enterobius vermicularis (Pinworm): Gravid female worms migrate to the perianal area at night to lay eggs. Causes intense nocturnal perianal pruritus. Diagnosed using the Scotch tape test to visualize oval eggs flattened on one side.
• Ascaris lumbricoides (Giant Roundworm): Fecal-oral egg ingestion. Larvae migrate through lungs causing eosinophilic pneumonitis, then swallowed into the small intestine. High worm burden causes intestinal obstruction or biliary tract blockages.
• Ancylostoma duodenale & Necator americanus (Hookworms): Microscopic filariform larvae in soil penetrate bare skin of feet, migrate to lungs, and enter GI tract. Adult worms attach to intestinal mucosa and suck blood, leading to severe microcytic iron deficiency anemia.
• Strongyloides stercoralis: Filariform larvae penetrate skin. Capable of autoinfection (larvae mature within intestine without leaving host). Diagnosed by finding rhabditiform larvae in stool.
• Trichinella spiralis: Transmitted by eating undercooked meat (pork or bear) containing encysted larvae. Larvae migrate into skeletal muscle, producing severe myalgia, periorbital edema, fever, and marked eosinophilia.

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Helminthic Pathogens: Cestodes (Tapeworms)

Cestodes are flat, segmented tapeworms composed of a head (scolex) and proglottids.

• Taenia solium (Pork Tapeworm):
  • Ingestion of encysted larvae in undercooked pork causes intestinal tapeworm infection.
  • Ingestion of T. solium eggs via fecal-oral contamination leads to cysticercosis / neurocysticercosis, where larvae encyst in human brain tissue, presenting with seizures and classic "Swiss cheese" brain parenchymal calcifications on MRI.
• Echinococcus granulosus: Ingestion of eggs from dog feces (sheep intermediate host). Causes hydatid cyst disease in the liver. Surgical aspiration or cyst rupture can release hydatid fluid, precipitating severe anaphylactic shock.

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Helminthic Pathogens: Trematodes (Flukes)

Trematodes are leaf-shaped flukes requiring freshwater snails as an intermediate host.

• Schistosoma mansoni & Schistosoma japonicum: Free-swimming cercariae in water penetrate human skin. Adult worms live in mesenteric veins. S. mansoni eggs possess a prominent lateral spine; S. japonicum eggs have a small round spine. Egg deposition in liver parenchyma induces granulomatous inflammation, pipe-stem periportal fibrosis, and portal hypertension.
• Schistosoma haematobium: Cercariae penetrate skin; adult worms inhabit the vesical venous plexus of the bladder. Eggs possess a prominent terminal spine. Causes painless hematuria, bladder inflammation, and is strongly linked to squamous cell carcinoma of the bladder.